A portapotty may be the simplest solution for you
Toilets do NOT flush into the bilge! The odor would be so gawdawful you couldn't be on the boat if they did. A marine toilet needs a 3/4" thru-hull and seacock for flush water...waste is flushed into a holding tank, which is emptied by pumpout through a deck fitting at a marina or dumped at sea (you must be at least 3 miles offshore from the nearest land to dump a tank legally) using an onboard manual diaphragm pump or electric macerator pump through another thru-hull. You'll find diagrams of typical head/holding tank installations here: http://www.westmarine.com/webapp/wcs/stores/servlet/WestAdvisorDisplayView?storeId=10001&langId=-1&catalogId=10001&advisor=436.htm Cost of all the materials needed is at least $400, plus labor if you don't do the work yourself. Because marine toilets and holding tanks require a considerable amount plumbing--intake hose, discharge hose from head to tank, tank pumpout hose, vent hose, at least one thru-hull and seacock, etc--and also a good bit of maintenance, and because a holding tank big enough to be worth the effort and expense to install it will take up quite a lot of the storage space on a 25' boat, the best solution for you may be a portapotty. Portapotties are are self-contained systems--the tank is directly under the bowl and a reservoir holds flush water--and are available in both portable models that require carrying the tank off the boat to dump it and models that can be permanently installed and fitted for pumpout. No plumbing is required for portable models...permanently installed models only need a vent line and a pumpout hose to a deck pumpout fitting. They can also be dumped at sea by installing a y-valve in the pumpout line and adding a hose and pump to go to a thru-hull and seacock. Because they use so little flush water, the larger 5-6 gal portapotties hold about as many flushes as 15 gallon holding tank connected to a manual marine toilet. Cost to install a portable portapotty: under $100. Cost to install a larger permanently installed model about $250. IMO, a portapotty--either type--the best solution for you right now. Inexpensive, simple to install, no moving parts, minimal maintenance. You have enough to learn, and most likely enough other expenses, that you don't need to spend $500 or more to install a toilet and holding tank. It's not an irrevocable decision...f you decide after a season or two that you'd rather change it to marine toilet and holding tank, you can.
